Reciprocating piston engines

ABSTRACT

A reciprocating piston engine has a cylinder with a transfer port and an exhaust port, wherein the transfer port and exhaust port are at least partially coincident and are provided with a port valve. The engine includes a pump having a divided chamber therein, one side of the chamber being connected to a crankcase via a connecting port, and the other side of the chamber having an inlet port, an outlet port, and a valve to ensure unidirectional flow therethrough. The chamber is divided by a moving member responsive to variations in pressure in the crankcase to cause flow through the other side of the chamber.

BACKGROUND OF THE INVENTION

The present invention relates to an engine and in particular, althoughnot exclusively, to a two-stroke reciprocating piston engine.

It is known to provide a crankcase-scavenged two-stroke enginecomprising a piston which reciprocates in a cylinder, the cylinderhaving transfer ports from the crankcase to the cylinder, and an exhaustport. The top of the exhaust port is located higher up the cylinder thanthe transfer ports, so as to permit most of the combustion gases toescape before a new charge enters the cylinder via the transfer ports.In other words the exhaust port is uncovered by a descending pistonbefore the transfer ports. A subsequent charge enters the crankcase onthe upstroke of the piston, and is pushed into the cylinder when thetransfer ports reopen on the next down stroke of the piston.

Several problems are associated with the prior art crankcase-scavengedtwo-stroke engines. The requirement for the transfer ports to be in theswept stroke represents an inefficiency of the induction cycle sincelittle or no work can be obtained from the piston displacement when thetransfer ports are open.

It is well known that fresh charge can pass directly to exhaust, and ithas been proposed to provide a tuned exhaust in an attempt to push theescaped charge back into the cylinder by the use of pressure pulses butthis can result in an engine with a narrow power band.

The exhaust and transfer port design of prior art two-stroke engines istypically a compromise which may reduce the theoretical maximum poweroutput from the engine and may also contribute to increased emissionsfrom the engine.

These problems are all well known, and numerous solutions have beenproposed to improve engine efficiency, to reduce contamination of thecharge due to crankcase lubricant and to reduce pollution due tounburned fuel leaving the exhaust port.

What is required is an improved engine which can overcome theaforementioned problems, and maximise the opportunity for charge pumpingand charge compression, and reduce transfer losses in a simple and costeffective manner.

SUMMARY OF THE INVENTION

According to the invention there is provided a two-stroke engine havinga cylinder with a transfer port and an exhaust port, whereby thetransfer port and the exhaust port are at least partially coincident,the transfer port and the exhaust port being further provided with aport valve, operable between a position to substantially close thetransfer port and open the exhaust port during an exhaust phase of theengine, and a position to substantially open the transfer port and closethe exhaust port during a transfer phase of the engine.

In this arrangement the fresh charge is substantially prevented fromexiting the cylinder through the exhaust port. Furthermore thisarrangement allows the transfer port to remain open for longer whencompared to prior engines since the transfer port opens into the exhaustport, and thus remains open until the top of the exhaust port is closedby a piston of the engine. This allows an increased volumetric charge tothe combustion chamber to provide an increase in the power output fromthe engine. Correspondingly the engine may have improved overall engineefficiency.

In one embodiment the valve may further comprise a deflector to deflectan incoming charge radially inward to the cylinder.

An engine so arranged has an improved swirl of the charge introducedinto the cylinder when compared to the prior engines since the freshcharge introduced into the cylinder enters the cylinder radially inwardand away from the exhaust port.

In one embodiment the deflector is arranged to deflect the incomingcharge to one side of the centre and towards the top of the cylinder.This has the effect of producing an upward helical swirl of the charge.

In one embodiment the engine is further provided with a fuel injector toensure an accurate fuel/air ratio over a wide range of operatingconditions. In this embodiment the transfer port is arranged to inputfresh air to the cylinder from either a crankcase of the engine or froma separate air pump. The transfer port may be in fluid communicationwith a turbo charger or supercharger to provide additional boost.

Preferably a low friction port valve is used to reduce parasitic lossesto a minimum. In the preferred embodiment the port valve comprises arotary valve which may be operable by an electric motor, typically inconjunction with a conventional engine management system. Alternativelythe port valve can be operated from a direct drive of the engine such asfrom a flywheel having a cam profile thereon adapted to operate thevalve.

In yet a further alternative the port valve is resiliently biased, byfor example a suitable spring, whereby the port valve is arranged to beopened by transfer gases from the engine, in use.

In another embodiment the transfer port is provided with a transfertract with a transfer valve such as a reed valve to ensureunidirectional flow of gaseous fluid through the transfer port. Thisarrangement prevents combustion gases entering the transfer tract, andthus ensure that a fresh charge is not contaminated.

An arrangement of the port valve and transfer valve so described permitsthe full stroke of the piston to be utilised to compress the chargesince no charge escapes from the combustion chamber via the exhaust portor the transfer port during charge compression.

It will be appreciated that the port valve and the transfer valve can beadjusted to have variable timing depending on the rotational speed ofthe engine, the position of the piston within the cylinder and the poweror torque demand. The valves may also be adapted to be partially orprogressively opened or closed. Such variable timing enables tuning ofthe engine for optimisation of the power output or the fuel efficiency,or for controlling emissions from the engine. The advantage of suchprogressive valve timing is that an incoming charge from the transferport can be used to create a swirl to push the combustion gases from thecylinder after combustion of a previous charge.

In the preferred embodiment the invention is adapted for a singlecylinder engine. However a multi-cylinder engine may also benefit fromthe invention provided that the exhaust port and transfer port of eachcylinder is provided with a port valve, one for each piston/connectingrod assembly.

In an alternative arrangement there is provided a two-stroke enginehaving a cylinder with a transfer port and an exhaust port, the exhaustport being further provided with a port valve operable between aposition to substantially open the exhaust port during an exhaust phaseof the engine, and a position to substantially close the exhaust portduring a transfer phase of the engine.

In this arrangement the transfer port and the exhaust port are notrequired to be coincident and conventional transfer ports can be used totransfer a fresh charge into the cylinder.

The invention also provides a reciprocating piston engine assemblyincluding a cylinder with an inlet and an exhaust, a crankcase, a cranka connecting rod and a piston, the crankcase comprising a closed chamberhaving a connecting port in a wall thereof, and the assembly furthercomprising a pump having a divided chamber therein, one side of saidchamber being connected to said crankcase via said connecting port, andthe other side of said chamber having an inlet port, an outlet port, andvalve means to ensure unidirectional flow therethrough, wherein saidchamber is divided by a moving member responsive to variations inpressure in said crankcase to cause flow through said other side of saidchamber.

The rise and fall in crankcase pressure is an inevitable result ofpiston reciprocation, and the effect in the pump is to cause movement ofthe moving member, with consequent cyclical variation of the volume ofsaid other side. The valve means ensure that unidirectional flow is aresult, and consequently the pump can be arranged to provide a supply offresh clean air to the inlet tract of the engine. It will be appreciatedthat the moving member is a barrier to crankcase oil mist.

It will be appreciated that the usual transfer passages to the crankcaseare eliminated so that the full displacement of the piston is used togenerate a cyclical pressure variation in the crankcase, which can betransferred to the pump.

In addition, an engine so arranged reduces the unpowered displacement ofthe piston stroke, due to the transfer port being open in the prior atdesign, which may provide an increase in the power output from theengine. Correspondingly the engine may have improved overall engineefficiency.

In a multi-cylinder engine, the crankcase is divided into substantiallysealed chambers, one for each piston/connecting rod assembly.

In the preferred embodiment the pump provides clean air under pressureto the engine. A fan may be included upstream of the pump inlet port inorder to increase inlet pressure, and thereby outlet pressure.Sophisticated valving is of course possible, including variable valvetiming, and such an arrangement is particularly effective in scavengingof a two-stroke engine. In conjunction with an air inlet valve, theengine preferably uses fuel injection to ensure an accurate fuel/airratio over a wide range of operating conditions.

Air under pressure from the pump may also be mixed with fuel upstream ofthe engine, for example in a carburettor or indirect injection system.

In a further refinement of a two-stroke engine, air from the pump may beintroduced into the exhaust as a pulse to both urge burnt gases down theexhaust tract, and to prevent a fresh fuel/air charge from passing toexhaust before combustion, thereby mirroring the characteristics ofprior exhaust expansion chambers.

Preferably the pump has a first plenum chamber downstream thereof. Thisallows the fluid to be supplied for example to the exhaust or thecombustion chamber on demand and without pressure pulsing due to thecyclical nature of pump operation.

In the alternative embodiment pressure pulsing of the pump may be usedto advantage in a tuned inlet tract, so as to maximise the volume of airadmitted to the cylinder on each suction stroke.

The pump may be arranged separately from, immediately adjacent orintegrated in the crankcase. The separate location of the pump from theengine has the advantage that a cooler and thereby denser charge isprovided to the cylinder than prior engines using a convention transferport design. Any kind of moving member is possible, but preferably a lowfriction member is preferred so as to reduce parasitic losses to aminimum. In the preferred embodiment the moving member comprises abellows, the capacity of said bellows being substantially equal to theswept volume of the piston. In an alternative arrangement the movingmember is a diaphragm.

Advantageously the inlet port of the pump is in fluid communication withan air box, the air box being open to atmosphere. In an alternativeembodiment the inlet port has a venturi with a fuel supply to provide acharge for the combustion chamber.

A fan may be included upstream of the pump inlet port in order toincrease inlet pressure, and thereby outlet pressure.

In accordance with another embodiment there is provided a second plenumchamber downstream of said first plenum chamber. The second plenumchamber operating at a higher pressure to introduce clean air into theinlet or exhaust at a higher pressure than the first plenum chamber.

The engine assembly may further include a second pump, said second pumphaving an inlet connected to an air box upstream thereof, and an outletconnected to the inlet of said first plenum chamber. The second pump maybe an engine driven pump.

In accordance with another aspect there is provided a reciprocatingpiston engine assembly having a flywheel, wherein the flywheel includesa cam profile thereon adapted to operate a reciprocating pump. Such apump may be used to supply clean air under pressure, for example to thefirst or second plenum chamber.

The combination of the port valve and the pump is particularlyadvantageous, and promises an engine which has an increased power outputand reduced harmful emissions when compared to prior engines. Theaddition of the transfer valve to this combination may further improvepower output, reduce harmful emissions and improve overall engineefficiency.

BRIEF DESCRIPTION OF THE DRAWING

Other features of the invention will be apparent from the followingdescription of a preferred embodiment shown by way of example only inthe accompanying drawing, in which;

FIG. 1 is a schematic representation of an engine according to thepresent invention prior to charge ignition.

FIG. 2 is a schematic representation of the engine of FIG. 1 undergoingexhaust.

FIG. 3 is a schematic representation of the engine of FIG. 1 undergoingcharge transfer.

FIG. 4 is a schematic representation of an engine according to anotherembodiment of the present invention.

FIG. 5 is a schematic representation of an engine according to yetanother embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 to 3 illustrate an engine according to the present invention,generally designated 10, completing one cycle. Like features are shownwith like reference numerals. FIG. 1 shows the engine 10 prior to acharge 48 being ignited. FIG. 2 shows the engine 10 undergoing exhaust.FIG. 3 shows the engine 10 undergoing charge transfer.

In FIG. 1 a piston 12 is shown which reciprocates in a cylinder 14. Thepiston 12 and the cylinder 14 together define a combustion chamber 16.The combustion chamber 16 is provided with a fuel injector 18, a sparkplug 20, a transfer port 22 and an exhaust port 24. The transfer port 22is coincident with the exhaust port 24 and has a valve 26 to permiteither the exhaust port 24 or the transfer port 22 to be open to thechamber 16. The valve 26 has a free edge 27 that is movable between thetop of the exhaust port 24 and the bottom of the transfer port 22. Thevalve 26 can be operated by any suitable means, such as an electricmotor, and may comprise a rotary valve.

The engine 10 of further comprises a crankcase 28 which defines acrankcase chamber 30. The crankcase 28 houses a crank 33, the crank 33being connected via a connecting rod 34 to the piston 12. The crankcasechamber 30 is in fluid communication with a pump 32 via a connectingport 34. The pump 32 has a membrane 36 that reciprocates in a pumpchamber 38. The pump 32 has an inlet 40 and an outlet 41. The inlet 40is in fluid communication with an air box (not shown) having an airfilter (not shown). The air box is open to atmosphere to provide asupply of clean and fresh air to the pump 32. Each of the inlet 40 andthe outlet 41 are provided with a one-way valve 42, 44, such as a reedvalve to permit unidirectional flow of fresh air through the pump. Theoutlet 41 is in fluid communication with the transfer port 22 via atransfer passage 46.

As the piston 12 reciprocates in the cylinder 14 the pressure within thecrankcase chamber 30 varies in a cyclic manner. This cyclic pressurechange causes the membrane 36 to reciprocate within the pump chamber 38.The one way valves 42,44 of the pump 32 allow the pump 32 to pump freshair in response to the varying pressure within the crankcase chamber 30.The membrane 36 acts to separate the volume of gas in the crankcasechamber 30 from the fresh air being pumped by the pump 32. This allowsthe oil contaminated gases within the chamber 30 to be separated fromthe fresh air being pumped by the pump 32.

In FIG. 1 the fuel injector 18 is shown injecting a fuel charge 48 intothe chamber 16. The charge 48 can be injected at any time after exhaustand between when the piston 12 is at bottom dead centre and before thepiston 12 reaches top dead centre in accordance with known techniques.In the Figure the piston 12 is shown as the top of the exhaust port 24is closed by the piston 12 moving up the cylinder 14 to compress thecharge 48. The valve 26 is shown in the position whereby the exhaustport 24 is closed. The inlet valve 42 is shown in the open position asfresh air is input to the chamber 38 due to the piston 12 moving up thecylinder 14.

Referring now to FIG. 2 the piston 12 is shown travelling down thecylinder 14 after ignition of the combustion gases and just after it hasuncovered the exhaust 24 so that the exhaust gases pass into the exhaust24. The valve 26 is shown in the position whereby the exhaust port 24 isopen and the transfer port 22 is closed. The exhaust gases are preventedfrom passing into the transfer passage 46 by the valve 26. The pressurein the crankcase 28 due to the position 12 travelling down the cylinder14 causes air within the crankcase chamber 30 to pass into the pump 32via the connecting port 34.

In FIG. 3 the engine 10 is shown undergoing charge transfer. The valve26 is shown in position to close the exhaust 24 so that the transferport 22 is open. The valve 26 also acts as a deflector so that fresh airfrom the pump 32 is deflected radially inward to the combustion chamber16 to provide an advantageous swirl and mixing with the charge 48 fromthe injector 18. The fresh air is directed radially inward to the centreof the piston 12 in an opposite direction to the exhaust gasesillustrated in FIG. 2. No charge 48 or fresh air escapes via the exhaustport 24 during charge compression since the exhaust port 24 remainsclosed by the valve 26. The piston crown may be shaped according toknown techniques to induce a desired swirl motion.

It will be appreciated that the valve 26 can be adjusted to havevariable timing depending on the rotational speed of the engine or theposition of the piston 12. Such variable timing permitting tuning of theengine 10 for optimisation of the power output or the fuel efficiency,or for controlling harmful emissions from the engine.

The injector 18 of FIGS. 1-3 may alternatively be omitted and the inletport 40 connected to an indirect fuel injection system or a carburettorin order to pump a fuel/air mixture. Furthermore the air or fuel/airmixture may also be thermally insulated from the engine to provide acooler and, therefore, denser charge.

An engine so described in FIG. 1-3 allows the transfer port 22 to remainopen for longer when compared to the prior art engine, which may providean increase in the power output from the engine. Correspondingly theengine may have improved overall engine efficiency, power output andpetrol consumption combined with a reduction in harmful exhaustemissions. An engine so constructed may also be cheaper to manufacturesince the complexity of the cylinder casting and internal transfer portsis reduced when compared to a prior two-stroke engine. Correspondinglythe tooling to manufacture the cylinder 14 is cheaper.

In an alternative embodiment the exhaust port and the transfer port arenot coincident and the port valve is operable to substantially open andclose the exhaust port only. In this arrangement transfer tracts andports of a conventional kind are used to transfer a fresh charge to thecylinder. The transfer tracts may be provided with one way valves suchas reed valves to ensure unidirectional flow therethrough.

FIG. 4 shows a schematic representation of a two-stroke enginedesignated 110. The engine 110 comprises a crankcase 112 which defines acrankcase chamber 114. The crankcase 112 houses a crank 116, the crank116 being connected via a connecting rod 117 to a piston 118 whichreciprocates in a cylinder 120. The piston 118 and the cylinder 120together define a combustion chamber 122. The combustion chamber 122 hasa fuel inlet 121, a fresh air supply 136 and an exhaust 125.

The crankcase chamber 114 is in fluid communication with a pump 124 viaa fluid connection 123. The pump 124 has a membrane 126 thatreciprocates in a pump chamber 128. The pump 124 has an inlet 130 and anoutlet 132. The inlet 130 is in fluid communication with an air box (notshown) having an air filter (not shown). The air box is open toatmosphere to provide a supply of clean and fresh air to the pump 124.Each of the inlet 130 and the outlet 132 has a one way valve (not shown)such as a reed valve. The outlet 132 from the pump 124 is in fluidcommunication with a plenum chamber, or pressure reservoir, 134. Anelectronic control valve 133 may also be provided between the pump 124and the pressure reservoir 134. The pressure reservoir 134 is in fluidcommunication with an inlet 136 to the combustion chamber 122 andoptionally an inlet 138 to the exhaust 125. The inlet 136 to thecombustion chamber 122 and the inlet to the exhaust 125 may also beprovided with electronic control valves 140 to regulate the flow offresh air according to the timing of the engine, and the inlet 136 mayconnect to inlet tract 22 of the embodiment of FIGS. 1-3, so as toreplace the transfer passage 46.

As the piston 118 reciprocates in the cylinder 120 the pressure withinthe crankcase chamber 114 varies in a cyclic manner. This cyclicpressure change causes the membrane 126 to reciprocate within the pumpchamber 128. The one way valves 130,132 of the pump 124 allow the pump124 to pump fresh air in response to the varying pressure within thecrankcase chamber 114. The membrane 126 acts to separate the volume ofgas in the crankcase chamber 114 from the fresh air being pumped by thepump 124. This allows the oil contaminated gasses within the chamber 114to be separated from the fresh air being pumped by the pump 124. Thepressure reservoir 134 acts as a source of pressurized fresh air whichis supplied to the inlet 136 and optionally to the inlet 138 to theexhaust 125.

Another embodiment of the present invention is presented in FIG. 5.Features common to the embodiment of FIG. 4 are shown with likereference numerals. In this embodiment there is provided a second plenumchamber or pressure reservoir 135 between the pressure reservoir 134 andthe inlet 138 to the exhaust 125. The second reservoir 135 may have anadditional inlet 137 which is connected to a second pump (not shown),for example an electric pump or an engine driven pump such as a camdriven pump. The additional inlet 137 and the inlet 138 to the exhaust125 may have electronic control valves 140. The second reservoir 135 isintended to operate at lower volume and higher pressure than the firstpressure reservoir 134.

It will be appreciated that the second pump of the embodiment shown inFIG. 5 may be used with the embodiment illustrated in FIG. 4. In thisinstance the second pump may be used to increase the pressure of thepressure reservoir (134).

The inlet 130 to the pump 124 of FIGS. 4 and 5 may alternatively beconnected to an indirect fuel injection system or a carburettor in orderto pump a fuel/air mixture. The carburettor or indirect fuel injectionmay alternatively be located downstream of the pressure reservoir 134 or135 on either or both of the inlet 136 to the combustion chamber or theinlet 138 to the exhaust.

The control valves 140 of the embodiments illustrated in FIGS. 4 and 5maintain an optimum pressure within the combustion chamber 122 dependingon the engine load or the engine speed. For example, the control valve140 on the inlet 136 may be used to provide additional combustion gasesto the combustion chamber 122 after the exhaust closes and beforeignition. The control valve 140 on the exhaust inlet 138 may be used topush unburned combustion gases back into the combustion chamber when theexhaust 125 is open to the combustion chamber 122. The inlets 136,138may also be aimed or introduced into the combustion chamber moreeffectively to assist with purging the unburned gasses.

An engine so described herein reduces the unpowered displacement of thepiston stroke, due to the transfer ports, which may provide an increasein the power output from the engine. Correspondingly the engine may haveimproved overall engine efficiency, power output, petrol consumption andexhaust emissions. Furthermore, since there is no engine oil mistintroduced into the charge the engine emissions may be reduced whencompared to the prior art crankcase-scavenged two-stroke engine. Thefull displacement of the piston is utilised in the pump 124. Furthermorethe air or fuel/air mixture may also be thermally insulated from theengine to provide a cooler and, therefore, denser charge.

An engine assembly so constructed may also be cheaper to manufacturesince the required casting of the cylinder 120 and internal transferports is reduced. Correspondingly the tooling to manufacture thecylinder 120 is less expensive.

Whilst a preferred embodiment for the device has been described it willbe appreciated that many other designs of the engine exist that wouldhave the desired effect of this aspect of the invention with the provisothat the variation in crankcase volume is used to pump atmospheric airinto the combustion chamber.

1. A two-stroke engine having a crankcase, a crank, a connecting rod, apiston and a cylinder with a transfer tract, a transfer port throughwhich a fresh fuel/air charge is fed into the cylinder, and an exhaustport, whereby the transfer port and the exhaust port are at leastpartially coincident, the transfer port and the being further providedwith a port valve, operable between a position to substantially closethe transfer port and open the exhaust port during an exhaust phase ofthe engine, and a position to substantially open the transfer port andclose the exhaust port during a transfer phase of the engine.
 2. Anengine according to claim 1, wherein the crankcase comprising a closedchamber with a connecting port in a wall thereof, and the assemblyfurther comprising a pump having a divided chamber therein, one side ofsaid chamber being connected to said crankcase via said connecting port,and the other side of said chamber having an inlet port, an outlet port,and valve means to ensure unidirectional flow therethrough, wherein saidchamber is divided by a moving member responsive to variations inpressure in said crankcase to cause flow through said other side of saidchamber, said outlet port being in fluid communication with the transferport of said cylinder.
 3. An engine according to claim 2, wherein saidpump is connected to a first plenum chamber downstream thereof, saidfirst plenum chamber having an inlet and an outlet.
 4. An engineaccording to claim 3, and further including a second pump, said secondpump having an inlet connected to an air box upstream thereof, and anoutlet connected to the inlet of said first plenum chamber.
 5. An engineaccording to claim 3, and further including a second plenum chamberdownstream of said first plenum chamber, wherein the second plenumchamber operates at a higher pressure than said first plenum chamber. 6.An engine according to claim 5, and further including a second pump,said second pump having an inlet connected to an air box upstreamthereof, and an outlet connected to an inlet of said second plenumchamber.
 7. An engine according to claim 1, wherein the port valvecomprises a rotary valve.
 8. An engine according to claim 1, wherein theport valve is resiliently biased whereby the port valve is arranged tobe opened by transfer the engine, in use.
 9. An engine according toclaim 1 wherein the transfer tract has a transfer valve to ensureunidirectional flow of gaseous fluid through the transfer port, in use.